The present disclosure relates to a method for enhancing a battery management system, for example for a rechargeable battery constructed from lithium ion battery cells or nickel metal hydride battery cells, to a battery management system, to a battery system and to a motor vehicle.
Batteries are being used more and more widely as a result of improved storage capacity, more frequent rechargeability and higher energy densities. Batteries with a relatively low energy storage capacity are used, for example, for small portable electronic devices such as mobile telephones, laptops, camcorders and the like, while batteries with a high capacity are used as an energy source to drive motors of hybrid or electric vehicles etc. or as stationary batteries.
Batteries may be formed, for example, by connecting battery modules in series, the battery modules also being connected in parallel in some cases and the battery modules in turn being able to consist of battery cells which are connected in series and/or in parallel.
Battery management systems are used for battery management, for example in order to fundamentally control modules, to increase the safety of batteries, to increase efficiency and to extend the lifetime of battery modules and battery systems composed of battery modules, in particular in motor vehicles, but not only there. One task of battery management systems is to determine the instantaneous state of charge or ageing state of the battery modules and the battery cells contained in the battery module, the battery state depending on the battery current and battery temperature of the battery cells or battery module.
The published patent application DE 10 2008 041 300 A1 describes a method for determining the actual state of charge of an electrical rechargeable battery. The method comprises: recording the rechargeable battery current supplied to the rechargeable battery and the rechargeable battery current flowing away from the rechargeable battery; providing a charge difference by integrating the rechargeable battery current over time; updating a measurement state of charge by adding the charge difference to the measurement state of charge; and deriving an estimated state of charge from a unique predefined association between states of charge and open-circuit voltages of the rechargeable battery on the basis of an open-circuit voltage of the rechargeable battery. Another method for determining the state of charge is known from the published patent application DE 10 2008 036 159 A1.
In order to increase efficiency and extend the lifetime, it is helpful to know the respective instantaneous power limits for charging and discharging the battery cell or battery module. It is then possible to prevent the cell or the module from being discharged or charged while contravening the power limits. This is because charging or discharging processes while contravening the power limit result in rapid ageing. In this case, the charging and discharging limits are each dependent on the instantaneous state of the battery cell/battery module.
In this case, the charging or discharging limit values can be determined in different ways. Their determination is based on functional relationships between the state of charge, battery current, battery voltage, battery temperature and battery power. For example, the power depends at least on the voltage and current, changes in the state of charge and in the temperature depend at least on the current and the voltage depends at least on the state of charge.
DE 33 17 834 C2 discloses a method in a control system for the traction motor of an electric vehicle with a traction battery, in which a discharge current target value predefined with the aid of an accelerator pedal is compared with two limit values and is limited, if necessary, in order to avoid overloading.